The present invention relates generally to avalanche photo-diode gain control circuits and more specifically to an active avalanche photo-diode gain control circuit for optimizing the gain of the avalanche photo-diode in an optical receiver usable in an optical measurement instrument, such as an optical time domain reflectometer.
Avalanche photo-diodes (APD) are used in optical receivers for converting an incoming optical signal into an electrical current signal. The current signal is coupled to a transimpedance or logarithmic amplifier for amplification. The output of the amplifier may be coupled to an analog-to-digital converter (ADC) for converting the analog signal into equivalent digital values at the sample rate of the ADC. The digital values may be stored in RAM associated with a digital controller, such as microprocessor, that performs averaging and other operations on the digital data to decrease noise and increase the signal-to-noise (SNR) of the receiver.
One of the APD's most important parameters is the reverse bias voltage associated with breakdown. When operated below breakdown, increases in reverse bias results in signal amplification. As the APD bias increases, the gain and shot noise also increases, but within manageable bounds. This is the region of normal APD operation. However, at the breakdown voltage, dark currents increase exponentially, along with the APD's excess noise and the dark current shot noise. The result of the APD going into breakdown is saturated noise in the optical receiver output, and possible damage or destruction of the APD.
The internal gain of APDs is achieved by collision-induced generation of additional carriers. Because of the high field strength within the APD, photogenerated carriers create subsequent carriers in a cascading effect that results in multiplied photocurrents. The multiplication of photocurrents is given approximately by the equation: ##EQU1## FIG. 1 illustrates the gain curve for a typical InGaAs APD with a breakdown voltage of 72 volts. The horizontal axis is in volts and the vertical axis is the logarithm (base 10) of the signal gain given by equation 1. As shown in the figure, the gain remains relative constant until it approaches to within a few volts of the breakdown voltage.
If the gain mechanism in an APD was perfect it would multiply the signal and the signal shot noise without adding any excess noise of its own. However, APD gain is not perfect. Besides multiplying the signal and signal shot noise, the APD also adds noise. A statistical statement about the APD's average gain is that on average, each photo-generated carrier leads to the generation of G carriers at the end of the multiplication process. However, on an individual basis, any one carrier that initiates an avalanche may produce more or less than G carriers. This statistical variation in gain for individual photo-generated carriers adds noise to the final multiplied signal. The noise added by the multiplication process is characterized by the excess noise factor. When the multiplication is initiated by electrons, the excess noise factor is given by the equation: ##EQU2## In equation 2, k is the ratio of the electron ionization coefficient (.alpha.) divided by the hole ionization coefficient (.beta.) in the detector avalanche region. Typical values for k are 0.02 to 0.04 for silicon, 0.7 to 1.0 for germanium, and 0.3 to 0.5 for InGaAs with InP multiplication regions. FIG. 2 illustrates a typical curve for excess noise in an InGaAs APD. The breakdown voltage is 72 volts and k equals 0.3. The excess noise remains relative constant until within a few volts of the breakdown voltage and then increases very rapidly.
FIG. 1 shows that setting the APD bias close to the breakdown voltage increases the gain of the APD. However, the APD breakdown voltage is a function of temperature, which makes it difficult to set the bias on the APD for optimum gain. To guarantee the APD does not go into breakdown when the optical receiver is operated in a varying temperature environment, such as in optical measurement instruments like an optical time domain reflectometer, the APD bias is set well below the breakdown voltage at room temperature. Setting the bias voltage in this manner assures that the APD will not go into breakdown when the optical receiver is operated over a wide temperature range. Even though the APD is set well below breakdown, the temperature of the optical receiver needs to be monitored to track changes in the breakdown voltage. However, the temperature calibration of the thermistor used to measure the temperature within the optical measurement instrument has some uncertainty. Additionally, the change in breakdown voltage per degree C. of temperature change, q, is an estimated quantity having a relatively large uncertainty. If the uncertainty of q is .DELTA.q and the temperature change is .DELTA.T, then the bias on the APD needs to be set .DELTA.T.times..DELTA.q volts below the breakdown voltage to ensure the APD does not exceed the breakdown voltage. For example, the value of q for a NDL5551P InGaAs APD, manufactured and sold by NEC, Corp., is 0.2%/.degree.C. with a typical breakdown voltage of 70 volts resulting in a value of q=0.14 volts/.degree.C. The manufacturer does specify a tolerance for q but empirical data shows that the tolerance is about .DELTA.q=0.07 volts/.degree.C. In the TFS3031 Mini Optical Time Domain Reflectometer (OTDR), manufactured and sold by Tektronix, Inc., Wilsonville, Oreg. and the assignee of the present invention, the maximum operating temperature for the instrument is listed at 45.degree. C. ambient. This equates to a temperature change of 30.degree. C. with the assumption that the inside fo the instrument is several degrees warmer than the outside temperature after the instrument is warmed up. The possible uncertainty in the APD bias is 30.degree. C..times.0.07 volts/.degree.C.=2.1 volts. The maximum temperature differential for cold operation is even greater at about 40.degree. C. As a result the wworst-case uncertainty is about 2.5 volts. Thus, even with temperature tracking and adjustment, the APD bias must be set at about 2.5 volts below breakdown to guarantee the APD will not go into breakdown. Preventing the APD from going into breakdown is a significant requirement in OTDR instrument design because of the possibilities of instrument damage and complete instrument failure if the APD goes into breakdown.
One way to lower the uncertainty in setting the bias voltage of the APD is to provide thermal cooling for the APD. This decreases the temperature range, .DELTA.T, over which the APD operates. However, the uncertainty of the thermistor temperature calibration and the uncertainty of the APD q tolerance value still limits how close the bias on the APD can be set to the breakdown voltage. It also requires additional cost and circuit complexity to thermally cool these devices.
Varying the bias on an APD in an optical measurement instrument, such as an OTDR is known in the art. U.S. Pat. No. 5,043,608, assigned to the assignee of the present invention, describes an avalanche photo-diode non-linearity correction circuit for cancelling a portion of a non-linear recovery error response by the APD to an input signal. The gain of the APD is varied from data acquisition cycle to data acquisition cycle using a bias voltage control circuit with the acquired data being stored. The stored data from consecutive acquisition cycles are combined in a summation circuit to effectively cancel a significant portion of the non-linear recovery error response.
U.S. Pat. No. 5,491,548, assigned to the assignee of the present invention, describes a wide dynamic range optical receiver for use in an optical measurement instrument, such as an OTDR, were an APD is coupled to one or both low and high sensitivity signal channels. A voltage biasing circuit provides the biasing voltage on the APD and is controlled by inputs from a controller. The OTDR acquires waveform data over segments of the fiber under test with gain of the APD being set at different levels for each waveform segment. While these prior designs used variable APD gain to cancel non-linearity recovery error responses and increase the dynamic range of an optical receiver, they did not achieve the optimum gain from the APD due to the uncertainty limitations of preventing the APD from going into breakdown.
What is needed is an active avalanche photo-diode gain control circuit for optimizing the gain of the avalanche photo-diode in an optical receiver and a method for setting the bias on the avalanche photo-diode for optimizing the gain of the APD without risk of exceeding the breakdown voltage of the APD. The gain control circuit and method should bias the APD for the highest APD gain without adding so much noise to the output of the optical receiver that the APD noise dominates the system noise. Further, when used in an optical time domain reflectometer, the gain control circuit and method should increase the dynamic range or signal-to-noise ratio of the OTDR.